A study of protein-lipid interaction is essential to an understanding of the structure and function of biological membranes. The complexity of natural membranes, however, provides a formidable barrier to such investigations. It is therefore likely that the study of protein-lipid interaction in a well-defined system will provide both tools and insight necessary to pursue the more general aspects of this problem. For this reason, we intend to continue our study of the interaction of cyclopropane fatty acid synthetase from E. coli with its substrate, the unsaturated fatty acid moiety of a vesicular phospholipid molecule. We propose to prepare the enzymes in sufficient quantity to undertake well controlled physical studies of this system. The studies will largely involve analysis by 13 C nuclear magnetic resonance and therefore require the production of 13 C enriched lipids to attain adequate sensitivity. 13 C spectra will provide a means of monitoring changes in hydrocarbon chain conformation, mobility, and environment on the interaction of the lipid with the enzyme. The observations should provide a basis for analysis of protein-lipid interactions in biological membranes. We also propose a genetic analysis of two protein-lipid systems, E. coli pyruvate oxidase and bacteriophage PR4, a lipid containing bacteriophage which infects E. coli. Either or both of these may provide a system for the genetic modification of proteins which interact with lipid.